Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR valve may be controlled to achieve a desired intake air dilution for the given engine operating conditions. Traditionally, the amount of low pressure EGR (LP-EGR) and/or high pressure EGR (HP-EGR) routed through the EGR system is measured and adjusted based on engine speed, engine temperature, and load during engine operation to maintain desirable combustion stability of the engine while providing emissions and fuel economy benefits. EGR effectively cools combustion chamber temperatures thereby reducing NOx formation. Also, EGR reduces pumping work of an engine and increases compression ratio resulting in increased fuel economy. A diagnostic procedure may need to be periodically or opportunistically carried out to monitor operation of the EGR system.
Various approaches are provided for diagnostics of an EGR system. In one example, as shown in U.S. Pat. No. 5,508,926, Wade discloses a method for detecting restrictions in the EGR system during steady-state engine operations. Air pressure in the engine intake manifold is monitored over a test period while an amount (determined based on engine operating conditions) of EGR is delivered to the intake manifold. Changes in the monitored air pressure are filtered through a lag filter process comprising a dynamic filter coefficient. The filtered air pressure is then compared to a dynamic threshold to determine presence of a restriction in the EGR system.
However, the inventors herein have recognized potential issues with such systems. As one example, by relying on a single dynamic threshold for EGR system diagnostics, it may not be possible to differentiate between degradations in the EGR system causing insufficient EGR flow and those causing excessive EGR flow. Also, the approach of Wade may not be able to detect undesired EGR flow occurring when EGR is not requested. As another example, the approach of Wade is limited to steady-state conditions. However it may be desired to also perform EGR diagnostics during transient engine operating conditions so as to reduce tailpipe emissions.
In one example, the issues described above may be at least partly addressed by a method for an engine comprising: as commanded EGR flow changes, dynamically adjusting upper and lower EGR limits based on the commanded EGR flow, indicating EGR system degradation based on a ratio of accumulated difference between a measured EGR flow and one of the upper and lower EGR limits to accumulated commanded EGR flow relative to a threshold, the threshold based on exhaust NOx levels, and adjusting EGR flow based on the indication of degradation. In this way, by dynamically updating an EGR tolerance band based on commanded EGR flow, and correspondingly adjusting a threshold for EGR system degradation detection based on emissions levels, EGR system diagnostics may be effectively carried out during both steady-state and transient engine operating conditions.
As one example, a diagnostic routine of the EGR system may be carried out periodically or opportunistically during a vehicle drive cycle. As engine operating conditions change, a commanded EGR flow may be varied to meet the changing EGR demand. A dynamic EGR tolerance band may also be computed, the tolerance band having a lower limit and a higher limit, each based on the commanded EGR flow. In one example, the tolerance band may be calculated as a function of the commanded EGR flow using a multiplier whose value changes with change (e.g., magnitude of change and rate of change) in EGR flow, such as based on whether there is an increase or decrease in the commanded EGR flow rate. An actual EGR flow rate is measured via an EGR flow sensor, such as a pressure sensor (either an absolute pressure sensor or a delta pressure sensor). If the measured EGR flow rate is higher than the commanded flow rate, a difference between the higher limit of the tolerance band and the measured EGR flow rate may be estimated to obtain a mass flow error. If the measured EGR flow rate is lower than the commanded flow rate, a difference between the lower limit of the tolerance band and the measured EGR flow rate may be estimated to obtain the mass flow error. If EGR flow is detected during conditions when EGR is not commanded (that is, commanded EGR flow rate is zero), a difference between a fixed EGR (upper) limit and the measured EGR flow rate may be estimated to obtain the mass flow error. The mass flow error and the commanded EGR mass flow may be accumulated over a duration (herein also referred to as the test period). The ratio of the accumulated mass flow error to the accumulated commanded EGR mass flow may then be compared to a threshold. Distinct thresholds may be applied for diagnosing insufficient EGR flow, undesired EGR flow, and excessive EGR flow events taking into account acceptable limits of tailpipe emissions in each case. Degradation of the EGR system may be indicated and a diagnostics code may be set if the ratio is higher than the specified threshold. Upon indication of degradation in the EGR system, further supply of EGR may be temporarily disabled by closing the EGR valve. In one example, where the diagnostics approach is used to diagnose a high pressure (HP) EGR system, in response to degradation of the HP-EGR system, EGR delivered via a low pressure (LP) EGR system may be increased.
In this way, by dynamically adjusting an EGR tolerance band based on the commanded EGR flow, and varying the threshold for detection of EGR system degradation based on emissions standards, undesired tailpipe emissions caused by degraded EGR system may be reduced. By selecting distinct thresholds for the diagnostics procedure based on the commanded EGR flow rate relative to the measured EGR flow rate, degradation of the EGR system resulting in insufficient EGR flow may be distinguished from degradation resulting in excessive EGR flow, and appropriately addressed. By estimating undesired EGR flow (when EGR is not commanded) based on an accumulated intake air flow, leaks in an EGR valve may be detected. The technical effect of dynamically computing the tolerance band for EGR mass flow error estimation taking into consideration changes in EGR flow is that erroneous indications of EGR system degradation due to transport delays between delivered and commanded EGR flow rates may be reduced. Overall, by enabling diagnostics of the EGR system to be carried out reliably and accurately, the fuel economy and emissions benefits of EGR may be extended over a wider range of engine operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.